Harvesters such as windrowers, tractors, and forage harvesters, have to operate effectively in normal and high-speed modes. Typical construction for such vehicles include front ground wheels mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame and rear ground wheels mounted on a respective caster. Each of the front ground wheels is typically driven by a respective drive motor which allows variable speed in both the forward and reverse directions such that steering of the tractor is effected by a differential in speed between the front wheels with the rear wheels following the steering in a castering action.
Conventional harvesters generally use dual path steering for both in-field operation mode and high-speed road transport operation mode. Dual path steering generally operates by varying the speed of the two front drive wheels in order to steer the harvester. The left wheel slows while the right wheel speeds up to turn left, while the right wheel slows and the left wheel speeds up to turn right. Combined with passively castering rear wheels, this enables the conventional harvester to perform zero radius spin turns in the field, which is desirable for optimum field efficiency and maneuverability. However, during high-speed road transport operation mode (e.g., at speeds up to 24.5 mph) dual path steering does not provide adequate steering stability. This is due to several factors, including variable ground drive motor/pump efficiency, lack of steering feedback to the driver, dynamics of the harvester which uses the front wheels to steer with no stabilizing effect provided by the rear wheels, combinations thereof, or the like.
FIGS. 1 and 2 show perspective and detailed views of a conventional windrower 10. The windrower 10 generally includes front wheels 12, 14 rotatably mounted to a frame 16, and a damping system 18 mounted to a rear axle 20 of the frame 16. The windrower 10 includes a cabin 22 configured and dimensioned to receive an operator, and having a plurality of controls for operation of the windrower 10, such as controlling a header (not shown) attachable to the front 24 of the windrower 10, controlling movement of the windrower in a forward direction 26, and controlling movement of the windrower in a reverse direction 28. The windrower 10 uses a mechanical hydraulic steering valve for steering the windrower 10.
At the rear 30, the windrower 10 includes casters 32, 34 rotatably mounted on opposing sides of the frame 16. The windrower 10 includes two independent caster wheels 36, 38 mounted to the respective casters 32, 34, one on the left-hand side and one on the right-hand side of the windrower 10. The casters 32, 34 are damped by horizontally mounted passive dampers 40, 42 (e.g., shock absorbers). The dampers 40, 42 mount at their base to arms 46, 47 mounted rigidly to the vehicle axis or axle 20. The rod end of the damper 40, 42 is mounted to a second arm 44, 45, which mounts rigidly to and rotates with the caster upright shaft 48, 50 (e.g., axis). Such mounting causes extension and retraction of the damper 40, 42 which, in turn, damps the oscillation of the caster wheel 36, 38.
As noted above, during high-speed road transport operation mode, the dual path steering does not provide adequate steering stability. Particularly, the rear wheels 36, 38 passively caster during high speed operation with all steering accomplished by varying the speed of the front ground drive wheels, and no directional stability is provided by the rear wheels 36, 38. The tendency for the rear wheels 36, 38 to shimmy during high speed operation necessitates the damping system 18 for the caster wheels 36, 38. The damping system 36, 38 uses a form of friction or viscous damper 40, 42 which can affect the directional stability due to the damping force not allowing the wheel 36, 38 to change direction as quickly as desired for turns. While shimmying is most pronounced during high speed road operation, it can also occur during field operation mode, which necessitates that any rear steering system also provide adequate caster shimmy damping for field speeds (e.g., up to 15 mph).